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DCAN 12: Planning Control for Hazardous Substances
Aggregate quantities

27. Consent is always required when the aggregate quantity of a hazardous substance is present in an amount equal to or greater than its controlled quantity. However, a consent may also be required even though the amount present is below the controlled quantity for that substance or category of substance. To ensure compliance with the COMAH Directive, the aggregation of all specified hazardous substances present at an establishment in amounts less than the controlled quantity for the individual substances or categories of substances, must be taken into account in determining whether a consent is required for some or all of them. (See Notes 4 and 5 of the Notes to Parts A and B of Schedule 3).
28. To establish whether a consent is required in these circumstances hazardous substances present in amounts less than their controlled quantities will be added together according to an addition rule (see Notes 4 and 5 of the Notes to Parts A and B of Schedule 3). Where a number of substances which represent a similar type of hazard, e.g., very toxic and toxic substances or explosive and flammable ones, are together present at amounts below the relevant controlled quantity for that substance or category of substance, the quantities present are expressed as partial fractions of their controlled quantities and added together. If these add to a sum of 1 or greater, then a consent is required for each of the substances which have been included in the addition. Substances with unrelated hazards are not added together, so toxic substances are not added to flammable ones. Two examples of how the addition rule applies are -
Example 1: The following substances are present together at an establishment
SubstanceAmount PresentControlled Quantity
Bromine21.00 tonnes20.00 tonnes
Chlorine3.00 tonnes10.00 tonnes
Hydrogen selenide0.5 tonnes1.00 tonnes*
Very toxic1.00 tonne5.00 tonnes
Toxic5.00 tonnes50.00 tonnes
Bromine is present in an amount greater than its controlled quantity and requires a hazardous substances consent. None of the other substances or categories of substance are present in amounts greater than their controlled quantities. But, they all have similar hazard characteristics. They are either very toxic or toxic substances and fall within categories 1, 2 and 10 of Part B of Schedule 3. They must therefore be considered together. Expressed as partial fractions the sum is as follows:
3/10 + 0.5/50* + 1/5 + 5/50 = 0.30 + 0.01 + 0.20 + 0.10 = 0.61
The sum of the addition is less than 1, so there is no need for a consent for any of these substances, other than Bromine. * (See paragraph 29 for an explanation of the controlled quantity used for this substance in this calcualtion).
Example 2 :
SubstanceAmount PresentControlled Quantity
Bromine15.00 tonnes20.00 tonnes
Chlorine3.00 tonnes10.00 tonnes
Hydrogen selenide0.5 tonnes1.00 tonne
Ethylene oxide2.00 tonnes5.00 tonnes
Propylene oxide1.00 tonne5.00 tonnes
Very toxic1.00 tonne5.00 tonnes
Toxic 5.00 tonnes50.00 tonnes
Oxidising3.00 tonnes50.00 tonnes
None of the substances present are at amounts greater than their individual controlled quantities. But substances which fall within common hazards groups have to be considered under the addition rule. Bromine, Chlorine, Hydrogen selenide and the very toxic and toxic substances have common hazards and fall within categories 1, 2 and 10 of Part B of Schedule 3. They are added together. Expressed as partial fractions the addition is :
15/20 + 3/10 + 0.5/50* + 1/5 + 5/50 = 0.75 + 0.30 + 0.01 + 0.20 + 0.10 = 1.36
The addition sums to greater than 1, so for each of the five substances the controlled quantity is deemed to be present and a hazardous substances consent would be required for each of them. Any consent granted by the Department will be in respect of the amount of the hazardous substance present and not for the controlled quantity deemed to be present.
Ethylene oxide, Propylene oxide and the oxidising substance also have common hazards. They fall within categories 3, 4, 5, 6, 7, 8 and 9 of Part B of Schedule 1 and they, too, are added together. Expressed as partial fractions the addition is :
2/5 + 1/5 + 3/50 = 0.40 + 0.20 + 0.06 = 0.66
Since the sum is less than 1, there is no need for a consent for any of these three substances.* (See paragraph 29 for explanation of the controlled quantity used for this substance in this calculation).
29. For the purpose of the addition rule, the controlled quantity is that listed in Column 2 of either Part A or Part B of Schedule 3 to the Regulations subject to the following exceptions. Where, for substances in Part A of Schedule 3, a separate, greater quantity is shown in Column 3 that quantity will be the controlled quantity only for the purposes of the addition rule. To maintain existing health and safety standards, the controlled quantities for some hazardous substances have been retained at quantities lower than those specified by the COMAH Directive for the same named substance or category of substance. An example is Hydrogen selenide (substance no. 35) where for consent purposes a controlled quantity of 1 tonne has been retained; the equivalent quantity in the Directive is 50 tonnes. To ensure the addition rule is applied uniformly and without unnecessary penalty to operators, the higher quantity specified in the Directive will apply to those substances where, for the purpose of consent, a lower controlled quantity has been retained.
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